1. Field of the Invention
The present invention is in the field of rotating cutting tools used for milling downhole metal members in a well bore, and rotating cutting tools used for drilling a well bore through an earth formation.
2. Background Information
Various milling applications and drilling applications have, over the years, suffered from the problem of a xe2x80x9cdeadxe2x80x9d spot in the center of the mill or drill bit. As the mill or drill bit rotates, it revolves around a central axis. At the point where that central axis passes through the cutting face of the mill or drill bit, the cutting structure is degraded and quickly becomes ineffective. Ultimately, a core, or depression, is worn into the cutting matrix. As the core wears further into the matrix, fluid circulation in the area is reduced, and cuttings resulting from the milling or drilling operation are no longer effectively removed. The reason for this problem is that on the cutting face, at the point where the central axis passes through the cutting face, the cutting elements have essentially a zero cutting surface speed.
In a typical milling situation, for instance, a segment of metal tubing may be stuck in the well bore. The tubing will usually be bent and leaning against the sides of the casing or well bore. In this situation, a rotating metal milling tool will typically be run downhole to mill away the bent metal tubing. As the milling tool progresses downwardly, milling away the bent tubing, there will be a number of times when the wall of the bent tubing is positioned against the center of the face of the milling tool. This results in a zero relative speed of the cutting elements across the bent tubing at the center point, with little effective cutting taking place. This generates considerable heat at the center point, which can soften the cutting matrix, leading to rapid deterioration of the matrix at the center point. Ultimately, this can create a deep depression or cone in the center of the face of the milling tool. When the depression deepens to the point of reaching the body of the milling tool, which is typically made of steel, no further milling progress can be made.
A similar problem can occur in the drilling of a well bore through an earth formation. Coning of the drill bit can occur at the center point, resulting in slowing or even stalling of drilling progress, requiring the drilling operation to be stopped until a new bit is installed. It is the object of the present invention to provide a design, which can be incorporated into either a milling tool or a drill bit, which will not have a zero cutting speed anywhere on the cutting face of the tool, thereby eliminating the coning problem and allowing a full depth milling or drilling operation to be accomplished.
Whether embodied in a milling tool or a drill bit, the tool of the present invention has a cutting assembly consisting of one or two cutting structures, with at least one of the cutting structures being rotated about an axis offset from the axis of the borehole. The tool is connectable to the lower end of a drill string or coiled tubing, for positioning in a well bore. Use of the term xe2x80x9cdrill stringxe2x80x9d herein is intended to include all types of tubular strings, including coiled tubing, where the context allows. The cutting assembly as a whole rotates about its longitudinal axis. Further, each of the cutting structures rotates about its own longitudinal axis. The longitudinal axis of at least one cutting structure is offset from, but parallel to the longitudinal axis of the cutting assembly, and this cutting structure spans the longitudinal axis of the cutting assembly. Therefore, as the cutting assembly rotates, the offset cutting structure rotates independently, insuring that the center point of the cutting assembly does not have a zero cutting surface speed. This prevents coning of the cutting structures at the center point. Where a second cutting structure is present in the cutting assembly, it can also have an offset axis, or its axis can coincide with the axis of the cutting assembly.
In one embodiment, the cutting assembly can be mounted on the lower end of a housing connected to a drill string or coiled tubing, with a first cutting structure being fixedly mounted to the housing and a second cutting structure rotatably mounted to the housing. The rotational axis of the first cutting structure coincides with the axis of the housing, while the rotational axis of the second cutting structure is offset from the axis of the housing. In this embodiment, the first cutting structure is rotated by rotation of the housing, while the second cutting structure is independently rotated by a drill motor mounted within the housing. Rotation of the cutting assembly as a whole is accomplished by rotating the drill string to rotate the housing and cutting assembly, or by rotation of the housing and cutting assembly with a drill motor. The cutting assembly can be centered on the axis of the well bore or casing within which the apparatus is positioned.
In a second embodiment, the cutting assembly can be mounted on the lower end of a drill motor connected to a drill string or coiled tubing, with each of two cutting structures being independently rotated by the drill motor. Independent rotation of the cutting structures with a single drill motor can be accomplished by use of a single input, dual output transmission. Rotation of the cutting assembly as a whole is accomplished by rotating the drill string to rotate the drill motor and cutting assembly, or by rotation of the drill motor and cutting assembly with a drill motor. As with the first embodiment, the cutting assembly can be centered on the axis of the well bore or casing within which the apparatus is positioned.
In a third embodiment, a drill motor is fitted with clamp-on eccentric stabilizers which offset the axis of the drill motor from the axis of the borehole or casing. The drill motor is connected to a drill string or coiled tubing. Where the drill motor is connected to a rotatable drill string, the eccentric stabilizers contact the walls of the borehole or casing. Where the drill motor is connected to coiled tubing, the motor and stabilizers can be located within a rotatable housing which essentially aligns with the borehole or casing axis. In either case, the cutting assembly consists of a single cutting structure driven by the drill motor. This cutting structure can be aligned with the axis of the drill motor, with the result that the cutting assembly is offset from the axis of the well bore or casing. In this embodiment, the single cutting structure is rotated by the drill motor, while rotation of the motor and cutting assembly as a whole is accomplished by rotating the drill string, or by rotating the motor and cutting assembly with a drill motor.
In any of the embodiments where rotation of the apparatus is accomplished by a drill motor, a second drill motor may be used, or a secondary drive off a single drill motor may rotate the apparatus.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: